Sleepy heads Locusts hold their breath while they rest, probably because they have switched their brain to 'idle', a new study by Australian researchers has found.

"It has been known for more than 50 years that some resting insects switch from continuous gas exchange, that is breathing normally, to discontinuous gas exchange cycles (DGCs) when they rest," says Dr Philip Matthews from the University of Adelaide.

"What we didn't know was why."

The insect respiratory system is a branching system of air-filled tubes called tracheae, which open to the air through small holes called spiracles. These draw in oxygen and pump out carbon dioxide produced during respiration.

"The system is very efficient at delivering oxygen to tissues, because insects need to be able to increase their metabolic rate by between 40 and 100 times (in the case of bees) when they go from resting to flying," says Matthews.

"It is one of the most aerobic systems known for its weight, but is hugely over-engineered for delivering oxygen to tissues during rest periods."

A 2005 study on moth pupae, which breathe discontinuously while they undergo metamorphosis, measured their internal oxygen levels during DGCs while breathing air containing different levels of oxygen.

It found that regardless of the level of oxygen in the air, internally the pupae maintained a low constant level. The researchers suggested discontinuous breathing may have evolved to protect the pupae's organs against damage from exposure to high oxygen levels when their metabolic rate is low.

Fine fibre optic probes

To determine whether this might apply to adult insects, Matthews and his colleagues implanted fine fibre optic oxygen probes into their tracheal systems of locusts (Locusta migratoria) and conducted a similar series of tests.

Reporting in the journal Royal Society Biology Letters, the researchers found that while the locusts' internal oxygen levels dropped, they never fell to the same levels as the pupae.

"This indicated that it wasn't guarding against oxidative damage," he says. "So we compared the tracheal systems of locusts and moth pupae.

"We found the locusts' tracheal system has double the volume of the moth pupae, which gives it a larger reserve of oxygen to draw on when they hold their breath."

Increasing oxygen levels in air breathed by locusts makes no difference, he says, because the critical carbon dioxide threshold is always reached before the moth's low oxygen level, which causes the spiracles to open and the locust to breathe again.

"Moth pupae have a small volume of air but large volume of body fluids to sequester carbon dioxide, which allows oxygen levels to fall much lower before the critical threshold is reached."

Sleepy heads

Matthews says he explored other reasons the DGC cycles might have evolved, including whether they reduce the amount of water lost during respiration, known as the hygric hypothesis.

"But then we would expect all desert insects would do it, which they don't, while many tropical insects from humid environments do."

Other animals do change the way they regulate their breathing pattern when they are dormant - or worse.

"Cockroaches will commence DGCs if their brains are chilled, which puts them to sleep, or if their heads have been removed altogether," Matthews says. "Dormant land snails and torpid golden-mantled ground squirrels also show a similar pattern.

"It seems that locusts switch off their brains, and set their respiratory regulation to idle while they sleep."